Lab- and pilot- scale evaluation of bacterial inactivation and reactivation influenced by UV-LEDs arrangements in continuous flow water disinfection reactors

Abstract

In the recent years, UV-LED has emerged as an alternative UV source due to its distinguish advantages. One of the primary benefits of UV-LEDs is their ability to facilitate fine-tuned regulation of UV radiation utilization in disinfection systems, which could increase the effective UV fluence in the reactor and thus improve the disinfection efficiency compared with conventional UV disinfection devices. This study evaluated the inactivation and reactivation performance of continuous flow UV-LED (265 nm) disinfection reactor with four different designs of UV-LEDs arrangement under both lab- and pilot- scale testing conditions. Lab-scale tests showed that UV-LEDs at the outlet position delivered a much higher E.coli disinfection efficiency compared with other three arrangements (UV-LEDs at inlet, center, and separated positions) under various of UV-LED outputs and flowrates. Mechanisms analysis indicated that the improved disinfection efficiency was because UV radiation was applied to the lower velocity region in the reactor, leading to a longer effective UV exposure time and thus a higher UV fluence generated. Sub-germicidal levels of UV irradiation prior to the high UV fluence region could also improve the overall disinfection efficiency for separated and outlet arrangement, with synergy values of 0.21 log (16.6 % of overall log reduction) and 0.53 log (21.4 % of overall log reduction) generated. The reactivation experiments showed that the inlet arrangement led to the highest photo repair rates. Dark repair was not affected by the different UV-LEDs arrangement due to the relatively slow dark repair rates. The key findings in the lab-scale tests were then validated in a small-scale (Total flowrate was 13.3 L/min) recirculatory aquaculture system (RAS) system for shrimp farming. The inactivation and reactivation results under pilot-scale tests were consistent with the lab-scale testing results, showing that UV-LEDs at outlet position provided an average total bacteria disinfection efficiency of 1.84 logs in an 8 week testing period, which was more than twice of that for the inlet position. After disinfection, samples from the outlet arrangement also exhibited at least 26.5 % less photoreactivation percentage compared with that when LEDs were at the inlet position. These findings proposed an optimal design of UV-LEDs arrangement for UV disinfection reactors and verified this design in pilot-scale testing condition. This information is important for future optimizations of UV LED disinfection system design.